Abstract-We report reactivity of silicon doped single walled carbon nanotube (Si-CNT) towards the small atmospheric gas molecules O 2 , CO 2 , SO 2 and NO 2 using density functional theory based on the numerical basis set method. The reactivity of gas molecules is explained with binding energy, band structure, charge density, and density of states. We found that the substitutional doping of silicon atom in CNT increases the binding energy as compared with pure CNT. The charge density analysis reveals the formation of sigma (σ) bonds between silicon and carbon atoms. Further, the band structure and density of states clearly illustrate the creation of extra state near the Fermi level and reduction in the band gap, which acts as a reactive center for adsorption of the molecules on Si-CNT. We have observed that the large value of adsorption energy shows the chemisorption between molecules and Si-CNT. Mulliken population analysis clearly reveals the charge-induced dipole interactions between the Si-CNT and molecules, which are responsible for chemisorption for gas molecules. The donor like impurity state generated in energy gap almost disappears after adsorption of all gas molecules excluding NO 2 . We further note that molecules accept the electronic charge from nanotubes and have significant influence on electronic structure near the Fermi level and are responsible for the increase in the p-type conductivity of tubes.
We investigate the hydrogen adsorption capacity of Na-coated carbon nanotubes (Na-SWCNTs) using first-principles electronic structure calculations at absolute temperature and pressure. A single Na atom is always found to occupy the hollow site of a hexagonal carbon ring in all the six different SWCNTs considered, with a nearly uniform Na-C bond length of 2.5 A. Semiconducting zigzag nanotubes, (8,0) and (5,0), show stronger binding energies for the Na atom (À2.1 eV and À2.6 eV respectively), as compared to metallic SWCNTs with armchair and chiral geometries. The single Na atom can further adsorb up to six hydrogen molecules with a relatively constant binding energy of À0.26 eV per H 2 . Mulliken population analysis shows that positively charged Na atoms with 0.82e charge transfer to nearest carbon atoms which polarizes the SWCNT leading to local dipole moments. This chargeinduced dipole interaction is responsible for the higher hydrogen uptake of Na-coated SWCNTs. The transition state search shows that the diffusion barrier of Na atom on the SWCNT between two adjoining C-C rings is 0.35 eV. We also investigate the clustering of Na atoms to find out the maximum weight percentage adsorption of H 2 molecules. At high Na coverage, we show that Na-coated SWCNTs can adsorb 9.2-11.28 wt% hydrogen. Our analysis shows that, although indeed Na-coated SWCNTs present potential materials for the hydrogen storage, care should be taken to avoid Na atoms clustering on the support material at elevated temperature and pressure, to achieve higher hydrogen capacity.
Density functional theory is used to investigate the adsorption properties of O2, CO2, SO2 and NO2 gas molecules on pristine carbon nanotube (CNT) and Si-doped carbon nanotube (Si-CNT). All molecules except NO2 are physisorbed, with essentially no charge transfer between the CNT and molecules. The electronic properties of CNT are sensitive to the adsorption of NO2 because of its chemisorption, while they are insensitive to the O2, CO2 and SO2 molecules. The weak binding of these molecules on CNT is due to formation of charge-dipole interactions. In case of Si-CNT, all molecules are chemisorbed to the Si-C bonds with appreciable adsorption energy and significant charge transfer. The density of state analysis shows that the additional state near the Fermi level due to doping of silicon is responsible for chemisorption of the molecules. Further, our theoretical results suggest that molecule-induced modification of the density of states close to the Fermi level might significantly affect the transport properties of nanotubes.
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